Context is that these addresses appear as single entries in a table cell, so the icon approach would not disrupt readability. I agree that typographical methods just get crazy--make the page look like a ransom note.

From your description of the context, my first thought was footnotes in brackets.[1][2]and so on.

Or structure the table (if possible) so the IP addresses are grouped by their definitions.

The numbers in brackets might be better than an icon. An icon is an image, so you'd have to deal with making the image accessible and is it worth the bother. You see, you can say that blind people are colorblind. ;)

Thanks, Karen. I'm inclined to go with an icon so I can attach a tool tip to it with the definition. Wouldn't that be more accessible to a completely blind reader since the tool tip would be embedded where the IP address is displayed rather than making them navigate to the footnote?

OK, I don't know whether the one is more accessible than the other. Again, the context. Do you have 20 items in two categories? If so, you can remember the two categories as you read the list of 20 items. More variants might be tricky. As tooltips, they might even obfuscate the readability. (Had to use the O word!) Here, I recalled bus schedules where a bus route might have several footnotes that are rather complicated, such as "stop not used on holidays", "stop only used on holidays", "stop moved 100 meters from March 1 to June 30 due to roadworkds", etc. As footnotes, they don't interrupt my reading if those particular stops are irrelevant to me.

Depending how many categories of items you want to represent, formatting with different borders could work.

Have a legend above\adjacent to the table e.g. dotted underline = type 1, dashed underline = type 2, solid underline = type 3. Of course, this only works for a small number of types I guess.

Just enclosing the ip address in a a span tag allows you to use the title attribute to create a tooltip, though this doesn't necessarily help screenreader users (neither does the formatting solution for that matter), but combined with (ugh) hidden text, could work.

I think the previous suggestions of icons, with suitable alt text, would be the better solution thinking about it.